1. Field of the Invention
The present invention relates to a door check for automatically closing an open door and, more particularly, to a continuous speed change type door check for closing the door at a continuously changing speed.
2. Description of the Prior Art
Most frequently practised in the prior art is an oil cylinder type door check in which a piston is slidably fitted in a cylinder having a spring and a working fluid or oil confined therein so that the spring may be energized when the door is opened whereas the flow resistance is utilized to provide a damping effect when the door is closed.
Another door check is proposed in Japanese Patent Publication No. 52-21810, for example. This is the so-called "mechanical door check", in which a spring is energized when the door is opened whereas its releasing force is speeded up by a speed up gear train composed of multiple spur gears when the door is closed, and in which the damping effect is established by the use of a mechanical (or centrifugal) governor. Also disclosed in Japanese Patent Publication No. 52-3227 is a mechanical door check which uses a coil spring. We also have disclosed another mechanical check in application U.S. Pat. No. 157,665.
The oil cylinder type door closer establishes the damping effect by the use of the flow resistance of the oil confined in the cylinder so that its damping effect will disperse depending upon the change in the temperature. Specifically, at a high temperature, the viscosity of the oil will drop to reduce the flow resistance so that the door closing speed is accelerated. At a low temperature, on the contrary, the flow resistance is increased to decelerate the door closing speed. As a result, the oil cylinder type door check of the prior art must have its door closing speed adjusted. Another defect of the oil cylinder type is that the confined oil will leak raising a durability problem. Since, moreover, there has to be prepared a casing which includes a cylinder capable of enduring high spring force and oil pressure, another problem is that the door check itself must have a large size and weight. This heavy weight will make it troublesome to mount the door check.
On the other hand, the mechanical door check is freed from the problems of the dispersion in the damping effect accompanying the temperature change and the oil leakage. However, the overall construction is enlarged because a predetermined speed up ratio cannot be attained unless the number of gears to be interposed between the spring and the governor is large. If the speed up gear train is composed of multiple spur gears, there arise other problems in that the gear train has to be elongated and that the noises of the gears are high when the door is opened or closed.